Medical devices are often used to facilitate the flow of material as, for example, in a ureteral stent used for drainage of urine from the kidney to the bladder, or in a vascular graft used to maintain blood flow.
Typically these medical devices have been made from durable, non-biodegradable materials such as metals, polyurethanes, polyacrylates, etc. These non-biodegradable, non-dissolvable medical devices typically must be removed via an invasive procedure after they have served their purpose, or they remain in the body indefinitely. For those devices which remain in-vivo, there are often medical complications such as inflammation and other foreign body responses.
Devices have also more recently been prepared from biodegradable materials such as polyesters, polyanhydrides, and polyorthoesters. In U.S. Pat. No. 5,085,629, the use of a biodegradable polyester terpolymer of lactide, glycolide, and epsilon-caprolactone in a ureteral stent is disclosed. In the '629 patent, biodegradable has been defined to include hydrolytic instability. These polymers undergo hydrolytic chain cleavage in the presence of water to form low molecular weight water soluble species. The polyesters have been reported to undergo hydrolysis throughout the thickness of the device simultaneously (homogeneous hydrolysis) while the polyanhydrides and polyorthoesters have been reported to hydrolyse from the surface (heterogeneous hydrolysis). There are several problems inherent to devices manufactured with these biodegradable materials. There is a significant loss of strength in the device prior to any significant weight loss. These devices may undergo failure into large pieces which may occlude the vessel in which they have been deployed. Biodegradable devices which undergo surface hydrolysis may eventually reach a thin skin configuration that may also lead to vessel occlusion. Semicrystalline biodegradable materials have also been shown to leave insoluble crystalline residuals in the body for very long periods of time.
Polysaccharide--metal salt systems have been used for many years in biomedical applications. In European Patent Application 0 507 604 A2, an ionically crosslinked carboxyl-containing polysaccharide is used in adhesion prevention following surgery. The ionically crosslinked polysaccharide of this invention is left in-vivo. No attempt to dissolve the material is made.
Hydrogels have been widely used in biomedical applications. In U.S. Pat. Nos. 4,941,870; 4,286,341 and 4,878,907, a hydrogel is used as a coating on an elastomer base in a vascular prosthesis. This hydrogel remains in-vivo. Kocavara et al in J. Biomed. Mater. Res. vol. 1, pp. 325-336 (1967) have reported using an anastomosis ureteral prosthesis prepared from a poly(hydroxyethyl methacrylate) hydrogel reinforced with polyester fibers. This prosthesis is designed to be left in vivo.
In U.S. Pat. Nos. 4,997,443 and 4,902,295, transplantable artificial pancreatic tissue is prepared from an alginic acid gel precursor, a matrix monomer, and pancreas cells with Ca.sup.2+ ions and a matrix monomer polymerization catalyst. The calcium-alginic acid is used to provide mechanical integrity to the mixture while the matrix monomer is polymerized after which the calcium-alginic acid is removed with citrate via calcium chelation to leave a porous matrix. This use of the chelate to dissolve the calcium-alginic acid takes place in vitro. The calcium-alginic acid functions as a processing aid not as a structural member in the final artificial tissue device.
Polysaccharide--metal salt hydrogels have also been used to prepare tiny gel capsules containing pancreatic islet cells for the production of insulin. These capsules have been shown by workers at the Veterans Administration Wadsworth Medical Center to effectively control insulin levels in diabetic dogs for two years (Scientific American, Jun. 1992, pp. 18-22). These capsules remain in vivo.
In U.S. Pat. No. 5,057,606 a method and article useful for preparing polysaccharide hydrogels is disclosed. These foamed and non-foamed gelled articles are prepared by mixing together a first component comprising a suspension of a water insoluble di- or tri-valent metal salt in an aqueous solution of a polysaccharide, with a second component comprising an aqueous solution of a water soluble acid optionally to include the water soluble polysaccharide. These gels remain in vivo.
The present invention eliminates the problems associated with the materials discussed above. Hydrolytic instability is not relied upon to facilitate dissolution. The devices of the present invention are disintegrated upon demand through application of an agent, which acts to remove ionic crosslinking species, which may be anionic (mono or poly) or cationic (mono or poly) in nature, via binding or displacement mechanisms. As used herein, the term "disintegration" includes both the breakdown of the device into small particulates as well as into water soluble components. Triggered disintegration eliminates the time uncertainty observed with bioerodible materials from one patient to the next. Methods for triggered disintegration include administering or triggering release of the disintegration agent through the diet, administering the agent directly onto the device in an aqueous solution, encapsulating the agent in the device, parenteral feeding, and enema. Disintegration occurs without significant swelling of the device.